Technical Field
The present invention relates to a system for the automated adjustment of a pressure specified or set by a respiration device, in particular a positive end-expiratory pressure and/or a maximum airway pressure. The invention also relates to a device for machine respiration that is provided with such a system for adjusting the set pressure, in particular the positive end-expiratory pressure and/or the maximum airway pressure.
Description of the Related Art
In today's common forms of machine respiration, breathing gas is supplied to the patient at a positive pressure. This is why the airway pressure or the alveolar pressure during respiration is greater than the pressure in the pleural gap surrounding the pulmonary alveoli at least during the inspiration phase. During the expiration phase, there is no pressure applied to the airway by the respiration device, with the result that the lung tissue relaxes and the airway pressure or alveolar pressure drops. This kind of positive pressure respiration may under certain circumstances have the effect that the pressure conditions in the respiratory tract and in the alveoli, respectively, at the end of the expiration phase become so unfavorable that there is a collapse of parts of the alveoli. The collapsed part of the lung volume then will have to be unfolded anew in the subsequent breathing cycle. The functional residual capacity of the lungs is severely compromised, so that the oxygen saturation decreases, and also the lung tissue is permanently damaged.
In order to prevent a collapse of alveoli at the end of the expiration phase, positive-pressure machine respiration usually is carried out using a so-called positive end-expiratory pressure, which usually is briefly referred to as PEEP. With this measure it is in many cases possible to achieve an improvement in oxygen saturation.
In respiration with PEEP, the respiration device permanently applies—that is, both during the inspiration phase and during the expiration phase—a predetermined positive pressure, the PEEP, to the airway. Thus, the PEEP is still applied also after the end of the expiration phase. The maximum airway pressure is applied at the end of the inspiration phase when the airway is subjected to the highest load by the respiration device.
Ideally, the PEEP should be set large enough so that, during the expiration phase, the alveolar pressure is not, or at least only so far, below the pressure in the pleural gap that the alveolar tissue does not collapse under the effect of the pressure in the pleural gap.
On the other hand, too high of a value of the PEEP may have negative effects, especially during the inspiration phase. For, the lung tissue can be overextended at very high airway pressures during the inspiration phase. The necessary restriction of the maximum airway pressure at the end of the inspiration phase to values at which overextension of the lung tissue does not yet occur moreover results, at a high PEEP, in a restriction for the possible tidal pressure, i.e., the pressure difference between the pressure at expiration and the maximum pressure at inspiration.
Moreover, numerous studies also point out that a high value of the PEEP may impede the return flow of venous blood to the heart, with corresponding negative effects on the cardiovascular system.
In clinical practice, pressures such as PEEP or maximum possible tidal pressure and maximum airway pressure, respectively, are set by physicians or nursing staff of intensive care units on the basis of given physiological parameters of a patient or on the basis of known therapeutic benchmarks such as the so-called “ARDSnet Guidelines”, see e.g., The Acute Respiratory Distress Syndrome Network, The New England Journal of Medicine, 2000. 342: pp. 1301-1308 or Grasso et al., American Journal of Respiratory Critical Care, 2007, 176: pp. 761-767. Such a setting usually is made in advance and is only sporadically readjusted by physicians or nurses.
Predetermined guidelines naturally are not suited to reflect the actual state of a patient, but merely give experience values. However, endeavors are being made to carry out the adjustment of pressures determinative for respiration, such as PEEP or maximum airway pressure, for each patient individually on the basis of the current state of the patient.
Known methods of deriving information concerning closed or overextended alveoli take considerable time during which patient respiration is not possible in a regular breathing cycle. This is the situation e.g., with static pressure/volume curves (PN curves) recorded with the aid of the “super syringe method”, as described e.g., by Brochard L., Critical Care, 2006, 10: pp. 156-158. Moreover, during respiration there are occurring quite often inhomogeneous lung conditions in which closing of alveoli is to be observed in individual regions of the lung, but not in other regions. It is even possible that, with unchanged respiration parameters over a breathing cycle, the alveoli close during the expiration phase in some regions of the lung, whereas the alveoli are overextended during the inspiration phase in other regions of the lung. The known methods are not suited to resolve such conditions.
The method of electrical impedance tomography (EIT) described e.g., in Bikker et al., Critical Care, 2010, 14(3), R100; Tanaka, H. et al., American Journal of Respiratory and Critical Care, 2004, 169(7), pp. 791 to 800, or U.S. Pat. No. 6,502,984 B1 basically makes available a method allowing to gain real time information on the condition of the lung, in particular on the opening and closing of alveoli or overextension of alveoli. EIT is a non-invasive method that can be carried out directly at the patient's bedside and provides spatially differentiated information with regard to different regions of the lung. It is possible by means of EIT to differentiate regions of the lung with pathological conditions from regions “operating normally”.
For example, EP 1 593 341 B1 suggests an EIT-based method of regionally determining the alveolar opening and alveolar closing of the lung. The effect to be exploited in this regard consists in that the change of an impedance signal gained by EIT, which is affected by the patient's respiratory movements, is larger in regions in which the lung has not yet collapsed than in regions with collapsed alveoli. For determining the change of the EIT impedance signal caused by respiratory movement, the distance between minimum and maximum values in the EIT impedance signal can be used for example.
Due to its large potential for obtaining information resolved in time and space with respect to the lung condition, there have been made numerous efforts to make EIT usable as a diagnosis instrument for assessing the course of lung diseases. There is also the wish to be able to use such information in mechanical respiration.
US 2010/0228143 A1 describes an apparatus and a method for determining functional lung characteristics of a patient on the basis of EIT signals. The EIT signals reflect a spatial distribution of the electrical impedance in a cross-section through the thorax of the patient. The impedance signals for the various EIT pixels distributed across the cross-sectional plane of the thorax are used for determining a global electrical impedance integrated over the entire cross-section. By way of the course of time of the global electrical impedance, a breathing cycle is identified and divided into various phases in time. In addition, the thoracic cross-sectional plane is divided into several regions, and in each phase of the breathing cycle and for each region, there is determined the ratio of the electrical impedance of the sub-region to the respective global electrical impedance. By way of the thus obtained time curves of the impedance ratio for the individual regions, it is possible to determine the intratidal gas distribution across the lung, i.e., the contribution of individual regions of the lung to respiration in the course of a respiration cycle. It is suggested to use this information also for determining various respiration parameters, such as e.g., the PEEP or the tidal volume.
Despite all investigations and efforts with respect to making EIT usable for the diagnosis of the lung condition, however, there have been no systems become available to the present day for controlling or regulating the pressures to be applied in mechanical respiration, which are capable of controlling the mechanical respiration by means of EIT signals in the sense of a closed-loop control largely without human intervention.